1. Field of the Invention
The present invention relates to a digital signal processing system provided in a modulator/ demodulator (below, modem) for use in a digital data transmission network. More particularly, it is relates to a digital signal processing system for compensating a base band signal or pass band signal received in the modem using a predetermined sampling frequency.
An output of the digital signal processing system is supplied to a personal computer, a facsimile apparatus and the like through the digital data transmission network.
2. Description of the Related Art
A digital signal processing technique is widely utilized in the field of information processing. In general, first, a digital signal is converted to a base band signal based on a Nyquist rate, second, the base band signal is wave-shaped by a filter and also converted to a pass band signal based on a modulation method in the modem, third, the pass band signal is transmitted to another modem through the analog or digital data transmission network, fourth, the pass band signal is converted to the original base band signal (demodulation), and last, the base band signal is recovered in the other modem.
The modem mainly has two functions, i.e., the modulation and demodulation as explained above. Further, the type of the modem can be classified into high speed modems and low speed modems based on the processing speed. Low speed modems are mainly used for frequency shift keying (FSK) at a data transmission speed lower than 1200 bps (bit per second). By contrast, high speed modems are mainly used for phase shift keying (PSK) or quadrature amplitude modulation (QAM) at a data transmission speed higher than 2400 bps.
In the demodulation process, it is necessary to compensate phase jitter distortion contained in the pass band signal, particularly, in a low speed modem.
One example of a digital signal processing system, "An Automatic Equalizing Method and System" by Noriaki Fujimura, is disclosed in U.S. Pat. No. 4,097,807. This method is used for a suppressed carrier data transmission system for automatically compensating amplitude for static and dynamic variations in the phase, delay and frequency characteristics of a transmission channel in a high speed data transmission.
As an another example, "Joint Equalization and Carrier Recovery Adaptation in Data Transmission Systems" by Falconer is disclosed in U.S. Pat. No. 3,878,468. This prior art shows a pass band system and base band system. Both pass band and base band systems can be adapted to a low frequency system as mentioned above.